If your first beer (like most are) was an extract beer, just assume that as long as you added everything you were supposed to, and didn't add anything extra or leave anything out, and you had the right volume in the fermenter, your gravity was exactly where it was supposed to be. Nice thing about extract brewing is that the sugars are essentially fixed and as long as the ingredients and the volume are right, you can't miss the gravity.
Now, if you started with a Brooklyn Brew Shop kit (which some certainly do), and were brewing all-grain from the start, then you may have been a little high or low, and there's really not a way to know (unless you were measure the ABV chemcially in a lab, but that's $$$$$) and calculate, and at that point why bother.